Mono-dispersed Functional Polymeric Nanocapsules with Multi-lacuna via Soapless Microemulsion Polymerization with Spindle-like α-Fe2O3Nanoparticles as Templates
The mono-dispersed crosslinked polymeric multi-lacuna nanocapsules (CP(St–OA) nanocapsules) about 40 nm with carboxylic groups on their inner and outer surfaces were fabricated in the present work. The small conglomerations of the oleic acid modified spindle-like α-Fe2O3nanoparticles (OA–Fe2O3) were encapsulated in the facile microemulsion polymerization with styrene (St) as monomer and divinyl benzene (DVB) as crosslinker. Then the templates, small conglomerations of OA–Fe2O3, were etched with HCl in tetrahydrofuran (THF). The surface carboxylic groups of the crosslinked polymeric multi-lacuna nanocapsules were validated by the Zeta potential analysis.
KeywordsCrosslinked polymeric nanocapsules Multi-lacuna Functional surface Soapless microemulsion Template Fe2O3spindle
Most recently, polymeric nanocapsules have attracted more and more attention because of their specific properties and applications, such as drug delivery [1–4], catalysts [5, 6], light-emitting diodes , probing single-cell signaling , self-healing materials , and so on.
By now, a variety of physical and chemical strategies have been developed for the preparation of polymeric nanocapsules such as template method [10–17], micelle method [18–21], emulsion polymerization [22–24], interfacial polymerization [25–27], and other methods [28–31]. In most of the polymeric nanocapsules reported, the single voided cavum in the nanocapsules were globose. Furthermore, the surface-modification procedure is important to introduce some functional groups onto their inner and/or outer surfaces so that the functional polymeric nanocapsules were achieved.
In the present work, we developed a facile strategy for the crosslinked polymeric multi-lacuna nanocapsules (CP(St–OA) nanocapsules) about 40 nm with carboxylic groups on their inner and outer surfaces. The spindle-like α-Fe2O3nanoparticles were organo-modified with oleic acid (OA) and the oleic acid modified spindle-like α-Fe2O3nanoparticles (OA–Fe2O3) formed the small conglomerations in water. Then the small conglomerations were used as the templates for the soapless microemulsion polymerization. The small conglomerations of OA–Fe2O3were encapsulated in the crosslinked polymer nanoparticles obtained. Then the crosslinked polymeric multi-lacuna nanocapsules (CP(St–OA) nanocapsules) with carboxylic groups on their inner and outer surfaces were achieved after the templates were etched with HCl in tetrahydrofuran (THF).
The raphidian nano-crystal α-Fe2O3(TR-708-5W) is obtained from Shangyu Zhengqi Chemical Engineering Co. Ltd., Zhejiang, China. It was dried in vacuum at 110 °C for 48 h before use.
Oleic acid (OA) and divinyl benzene (DVB) (analytical reagents, Tianjin Chemicals Co. Ltd., China) were used as received without any further treatment. Styrene (St, analytical reagent, Tianjin Chemicals Co. Ltd., China) was dried over CaH2and distilled under reduced pressure. Ammonium persulfate (APS, Tianjin Chemicals Co. Ltd., China) was re-crystallized from ethanol before use. The other reagents, THF, ethanol, and concentrated hydrochloric acid (HCl), used were analytical reagent. Double-distilled water was used throughout.
Soapless Emulsion Polymerization
After 100 mL water and 0.1 g α-Fe2O3was mixed and stirred with electromagnetic stirrer for 20 min, 0.10 mL (0.315 mmol) oleic acid (OA) and 0.0142 g (0.355 mmol) sodium hydroxide (NaOH) were added for another 15 min. Then 1.0 mL St, 0.20 mL DVB and 0.013 g ammonium persulfate (APS) (1.0%) were charged into the dispersoid. The mixture was heated to 70 °C and maintained the temperature for 10 h with electromagnetic stirring. Another 0.013 g APS was added after the first 5 h of the soapless microemulsion polymerization.
After the brown dispersoid was cooled to the room temperature, the brown product, α-Fe2O3encapsulated with crosslinked polymer nanoparticles (Fe2O3/CP(St–OA)), was separated by being centrifuged at 8000 rps for 10 min and dried at 40 °C under vacuum.
The crosslinked polymeric multi-lacuna nanocapsules (CP(St–OA) nanocapsules) were achieved by the etching of the small conglomerations of OA–Fe2O3encapsulated in the crosslinked polymer nanoparticles (Fe2O3/CP(St–OA)) by the following process: 0.10 g Fe2O3/CP(St–OA) nanoparticles was dispersed into 10 mL THF containing 1.0 mL concentrated hydrochloric acid (HCl) with ultrasonic irradiation. The solid content was separated by being centrifuged at 8000 rps for 10 min after being immersed for 24 h. The etching process was conducted for another time to remove the templates completely. After the remained products were washed with diluted HCl several times, they were washed to neutrality with water.
A Bruker IFS 66 v/s infrared spectrometer was used for the Fourier transform infrared (FTIR) spectroscopy analysis. The mean particle size of the Fe2O3/CP(St–OA) nanoparticles was conducted with BI-200SM laser light scattering system (LLS, Brookhaven Instruments Co., Holtsville, NY). The morphologies of the nanoparticles and nanocapsules were characterized with a JEM-1200 EX/S transmission electron microscope (TEM). The nanoparticles and nanocapsules were dispersed into water and deposited on a copper grid covered with a perforated carbon film. The Zeta potentials of the nanocapsules at different pH values were determined with Zetasizer Nano ZS (Malvern Instruments Ltd, UK).
Results and Discussion
Oleic acid (OA) with a formula of C18H34O2 (or CH3(CH2)7CH=CH(CH2)7COOH) is a monounsaturated omega-9 fatty acid found in various animal and vegetable sources. It is widely used as surfactant for the soapless seeds emulsion polymerization in the recent years [32–36]. It could form mono-molecular or bi-molecular layer on the surfaces of the inorganic nanoparticles via its carboxyl groups and the C=C group could copolymerize with the vinyl monomers. So it acts as an interlinkage between the inorganic cores and the polymer shells.
The TEM images of the CP(St–OA) nanocapsules were given in Fig. 1d. The multi-lacuna structures were found in the CP(St–OA) nanocapsules with particle size of about 40 nm. They were near mono-dispersed. Compared with the structures and particle size of the Fe2O3/CP(St–OA) nanoparticles, the voided cavum were in the center of the nanocapsules. It could be predicated that the crosslinked polymer shells had reset and shrank in the etching period in THF.
In summary, we developed a facile strategy for the preparation of the mono-dispersed crosslinked polymeric multi-lacuna nanocapsules with functional groups on the inner and outer surfaces via the soapless microemulsion polymerization technique. Their structures, surface functional group and particle size could be altered by changing the inorganic oxide nano-cores, the surfmer (polymerizable surfactant) used and the formula of the polymerization. The technique is expected to extend the applications of the polymeric nanocapsules.
This Project was granted financial support from China Postdoctoral Science Foundation (Grant No. 20070420756).